1. Field of Invention:
This invention relates generally to electromagnetic flowmeters, and more particularly to a flangeless flowmeter whose components are encapsulated within a cast epoxy resin body that defines the flow conduit of the meter to form a highly compact, low-cost unit that may be readily installed in a flow line.
2. Status of Prior Art:
Magnet flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104, 3,824,856, 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are located at diametrically-opposed positions with respect to the tube. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instanteous value of the voltage established at the electrodes.
Typical of commercially-available flanged electromagnetic flowmeters is that unit manufactured by Fischer & Porter Co. of Warminster, Pa., whose Model 10D1430 flowmeter is described in Instruction Bulletin 10D1430A-1 Revision 4. This meter consists of a carbon-steel pipe spool flanged at both ends and serving as a meter body. Saddle-shaped magnetic coils are fitted on opposite sides of the inner surface of the meter body, the magnetically-permeable pipe spool acting as a core or return path for the magnetic field generated by these coils.
Disposed at diametrically-opposed positions within the central portion of the meter body are two cylindrical electrodes that are insulated from the pipe, the faces of the electrodes being flush with the inner surface of the pipe and coming in contact with the fluid to be metered. Connected to these electrodes and housed in a box external to the pipe are calibration components and a pre-amplifier.
In installing a standard flanged magnetic flowmeter, the meter is interposed between the upstream and downstream pipes of a fluid line, each pipe having an end flange. The mounting flanges on the meter are bolted to the flange of line pipes. It is, of course, essential that the circle of bolt holes on the mounting flanges of the meter match those on the pipe flanges.
In a magnetic flowmeter, the flow tube is subject to the same fluid pressure as the line pipes. The flow tube must therefore be of a material and of a thickness sufficient to withstand this pressure, even though the strength of the flow tube is unrelated to its measuring function. This design factor contributes significantly to the cost of a standard meter. Existing meters of the above-described type which are made up of components that must be assembled are generally of substantial size and weight and quite expensive to manufacture.
To overcome the drawbacks of a flanged electromagnetic flowmeter, the 1978 Schmoock U.S. Pat. No. 4,098,118 discloses a highly compact unit interposable between the flanged ends of upstream and downstream pipes in a line to meter fluid passing through the line. In one embodiment disclosed in the patent the flowmeter is constituted by an outer ferromagnetic ring having a pair of electromagnet coils supported therein at opposed positions along a diametrical axis normal to the longitudinal axis of the ring, the longitudinal axis passing through the central flow passage of an annular pressure vessel.
The vessel, which is formed of high-strength insulating material, is molded within the ring and encapsulates the coils as well as a pair of electrodes disposed at diametrically-opposed positions with respect to the flow passage along a transverse axis at right angles to the coil axis to define a unitary structure. The unit is compressible between the end flanges of the pipes by bridging bolts that pass through bore holes formed in the pressure vessel or which lie outside the outer ring to encage the unit.
Also disclosed in the Schmoock patent is a flangeless flowmeter which dispenses with the outer metal ring and makes use instead of a cylindrical plastic spool that forms the flow conduit of the meter and is surrounded by a reinforcing metal sleeve. An insulating body of synthetic plastic material is molded about this spool and embedded in this body are electrode holders and electromagnets which cooperate with the spool. Also embedded in this insulating body are ferromagnetic straps that interconnect the cores of the electromagnets to form a magnetic return path therefor. This return path in the case of the embodiment having an outer ferromagnetic ring is provided by the ring itself.
A ringless flowmeter of the Schmoock type is relatively difficult and expensive to make, particularly since prior to molding of the insulating body, the electrode holders and the electromagnets are unsupported and means therefore must be provided to hold these components in their assigned positions during the molding operation.